The advent of optical devices using low temporal processors, has created a need for broadband light sources having a relatively high output and emitting relatively incoherent light signals. In a gyroscope for example, an optical fiber, typically a kilometer or more in length, is wound into a loop and a light signal is recirculated within the loop in both directions. Motion of the loop causes a phase difference between the counterpropagating light signals in accordance with the well-known "Sagnac" effect. This phase difference is then used to measure the gyroscopic rotation. According to the "Sagnac" effect, a rotation of the loop increases the effective path length of one of the counterpropagating light signals. A relative path length difference therefore results in the emerging light signals after circulation through the loop of the two counterpropagating light signals. This path difference is measured by a detector as a phase difference, which is thus indicative of the angular rate of rotation to which the gyroscopic system is subjected. It is desirable that the light signals injected into the loop have a low temporal coherence so as to avoid interference effects from Rayleigh back scattering. As the phase shift induced by rotation is relatively small, any interference effects may indeed substantially alter the phase difference measured by the detector and provide a false measurement of the actual rate of rotation of the loop. Disparity between theoretical predictions and actual results has also been attributed to other non-rotationally induced phase differences such as those associated with residual fiber birefringence. It has also been shown that the use of a broadband light source having low temporal coherence compensates for the Kerr effect. A disclosure of this discovery can be found in PCT Patent Application Serial No. 82/01542 filed on Nov. 1, 1982 and assigned to the assignee of this present application.
A broadband incoherent light source having a lower temporal coherence than laser sources but having a spatial coherence higher than regular thermal incoherent light source is therefore advantageous for introducing light into an optical device such as a gyroscope. Furthermore, it is preferable that these light sources be small and compact and have a low energy consumption.
Light-emitting diodes (LED's) have generally been used for launching light into an optical fiber. A LED emits light under application of current flowing therethrough. The light comes from photons of energy caused by hole-electron combinations. The diode is forward-biased from an external source. Details on the structure of LEDs used for launching a light signal into an optical fiber can be found in: Fiber Optics by Robert G. Seippel, Reston Publishing Company, Inc., Reston, Va., pp. 107-116. While a LED emits light that is essentially incoherent and therefore suitable for application in a gyroscope, the output of the light signal emitted by a LED is generally insufficient in intensity and makes difficult the detection of very low rotation rates. Furthermore, the spectrum of wavelengths of the light emitted by a LED is substantially temperature dependent, an undesirable effect for many optical fiber applications such as gyroscopes. Finally, the coupling of a LED to an optical fiber gyroscope is typically of a poor quality.
Superradiant LEDs have also been used to obviate the problem raised by the low energy output of regular LEDs. Although a superradiant LED represents a progress over ordinary LEDs, the light signal coupled into an optical fiber, preferably a single-mode optical fiber, from a superradiant LED is low. Moreover, the temporal coherence of a superradiant LED is not as low as with a regular LED. Furthermore, because the light emitted by superradiant LEDs is the result of band transitions, it is more susceptible to temperature dependence and therefore lacks the stability required for use in gyroscopes and other optical systems.
Semiconductor laser diodes such as Ga(Al)As diode lasers operating continuously at room temperature in the near infrared region emit high output light very suitable for use as light sources in optical systems. However, the temporal coherence of the light emitted by a semiconductor laser diode is typically very high and can cause undesirable effects in an optical system requiring low temporal light such as a gyroscope.
Other miniature broadband optical sources used so far in fiber optic gyroscopes are superluminescent diodes (SLDs). However, SLDs generally fail to satisfy the wavelength stability requirement, as their emission wavelength is very sensitive to temperature (300 ppm/.degree.C.) and optical feedback. In addition, they incur high coupling loss into single-mode fiber, yielding typically only a few mW of usable power. Commercially available superluminescent diodes also exhibit a short lifetime. Moreover, coupling to a single-mode fiber is hindered by the poor spatial coherence of superluminescent diodes. Consequently, none of the non-fiber light sources hereabove described can be considered optimal light sources for use in gyroscopes and other optical devices as they all fail the requirements regarding temperature sensitivity and wavelength stability.
An alternate possibility is the use of high gain fibers optically pumped to a sufficiently high level to generate a significant superfluorescent output via Amplified Spontaneous Emission (ASE) also referred to as superluminescence. Single-mode glass fibers doped with an active ion are good candidates for superfluorescence sources as demonstrated by the high optical gains that they can provide. High gain materials such as Nd:YAG in a fiber form are particularly advantageous in a doped fiber configuration. Doped glass fibers present, however, the desirable advantage of emitting light with a broader spectral range. Recent improvements in the nature of the host material used in doped fibers have allowed superfluorescence to occur in doped fibers without having to resort to high pumping light intensities. A theoretical analysis of ASE in doped fibers is disclosed in an article by Michael Digonnet: "Theory of Superfluorescent Fiber Lasers," Journal of Lightwave Technology, Vol. LT-4, No. 11, November 1986. This article is hereby incorporated by reference. An experimental device comprising doped fibers emitting light by ASE was also disclosed in the following two articles: "Superfluorescent Single Mode Nd:Fiber Source at 1060 nm," K. Liu, et al., Electronics Letter, Vol. 23, No. 24, November 1987, and in "Neodymium Fiber Laser at 0.905, 1.06 and 1.3 microns," Po, et al., Optical Society of America Annual Meeting, Seattle, Wash., October 1986. Both of these articles are hereby incorporated by reference.
Additionally, a light source was disclosed in U.S. Pat. No. 4,637,025 to Snitzer, et al., which utilizes the physical phenomenon of Amplified Spontaneous Emission. This patent is hereby incorporated by reference. The light source described in the aforementioned patent comprises a pump source coupled to a fiber doped with an active laser material. The light emitted by the pump into the optical fiber has an intensity sufficient to produce Amplified Spontaneous Emission in the doped fiber. The resulting emission exits at the one end of the doped fiber which is not coupled to the pump source. In an alternative embodiment disclosed in the aforementioned patent, the backward component of the resulting ASE laser emission is reflected onto a dichroic mirror located between the pumping source and the doped fiber and combined to the forward component.
Although the superradiant light source disclosed by Snitzer is an improvement over light sources heretofore used in gyroscopes, it has several disadvantages that are obviated by the light source of the present invention. In the ASE laser source disclosed by Snitzer, the resulting light signal has at least the same temperature dependence as the ASE source wherefrom it is emitted. As the temperature dependence of an ASE laser source may be relatively high for certain applications, the temperature dependence of the resulting ASE laser source disclosed by Snitzer may not be acceptable for certain applications. Furthermore, in the first embodiment disclosed in the Snitzer patent, the light emitted by the pump source is launched directly into the doped fiber. The forward component traverses the doped fiber once and is therefore amplified by a factor e.sup.G, G being the gain of the doped fiber. The backward component of the ASE light signal is fed back into the pump, thereby inducing resonance in the pump cavity and may alter the output of the pump source. In the second embodiment disclosed by Snitzer, a dichroic mirror is used which reflects the ASE signal. A dielectric mirror such as a dichroic mirror is typically formed of multiple dielectric layers stacked on the top of one another. These dielectric layers necessarily reflect a portion of the light impinging upon the surface of those layers and thus reflect some of the pumping illumination emitted by the source back thereto, thereby creating feedback in the cavity of the pumping source. This optical feedback reduces the power of the pump source and also produces fluctuations in the superfluorescence output power. Moreover, the pump light signal is launched through the reflector, which decreases the coupling efficiency of the pump source. Furthermore, a reflector necessarily allows some portion of light to be transmitted even when it is designed to reflect light at a particular wavelength. The interposition of a mirror between the pump source and the doped fiber reduces the feedback induced by the backward component into the cavity of the pump source but by no means eliminates such feedback.
The coupling between the pump source and the doped fiber in the Snitzer embodiments has other disadvantages that are all circumvented in the devices disclosed in the present application. In particular, the pump source in the Snitzer device is coupled to the doped fiber using a longitudinal, parabolic index, self-locating fiber lens or a transverse fiber lens or simply a spherical end on the core of the doped fiber. Optical coupling elements such as lenses or mirrors do not have a very good coupling efficiency. Whenever it is possible, coupling methods employing optical fibers only have been preferred. End-tapering of an optic fiber to be coupled to a pumping source, in particular, substantially increases the coupling efficiency between the pumping source and the optical fiber. This entails, however, that the optical elements to be coupled be made essentially of optical fibers. The coupling optics used in the Snitzer device, however, does not use optical fibers. The Snitzer device does not use the excellent coupling properties of optical fibers and is not even designed to be able to use them.
Thus, there is a need for a light source that emits light having low temporal coherence, high intensity, high spatial coherence, low temperature dependence and well adapted for use in fiber optic devices.